[[Python]]
 

Python

Man, there's too much to do and note. Logging some stuff to investigate later…

Would be nice to create a binary search in text files in Python. Maybe based on an answer deep in Reading Huge File in Python.

This Explanation of Python "Yield" also mentions (at the bottom) explanations for decorators and metaclasses.

Would be good to experiment with least-squares polynomial fitting in Python.

Data Analysis

Looks like I should dive into:

Template Files to Start With

You have some template files in svn:

/Code/python_templates/trunk/make_standalone_application.bat
/Code/python_templates/trunk/setup.py
/Code/python_templates/trunk/template_cron_job.py
/Code/python_templates/trunk/template_gui_app.py
/Code/python_templates/trunk/template_gui_app_with_worker_thread.py

For / Else (Nobreak)

Python has a For/Else keyword that should have been called, "nobreak."

Sorting

From an old note-to-self

import operator
 
rows.sort(key=operator.itemgetter(4))
# or
rows.sort(lambda x, y : x[4] == y[4] and cmp(x[2],y[2]) or cmp(x[4], y[4]))

…not like I could just find the same info at the Python wiki or anything. :-P

Prepopulating lists with objects

Remember when you lost a couple of hours thinking that the following line created a list of objects.

    l = [Obj()] * n

It doesn't. It creates a list of references to one object.

What you meant to write was this:

    l = [Obj() for _ in range(n)]

Linux script that takes either stdin or files

if __name__=='__main__':
    if len(sys.argv) < 2:
        # Process lines coming from stdin.
        while 1:
            line = sys.stdin.readline()
            if not line:
                break
            my_process_line( line.rstrip() )
    else:
        # Process lines of the files specified.
        for fname in sys.argv[1:]:
            if not os.path.exists( fname ):
                treat_argument_as_literal( fname )
                continue
            with open( fname, 'r' ) as f:
                while 1:
                    line = f.readline()
                    if not line:
                        break
                    my_process_line( line.rstrip() )

The With statement

Making code more beautiful with "with". (Also mentions yield.)

cProfile vs. line_profiler and kernprof

timeit

import timeit
 
def Use_a():
    pass
 
def Use_b():
    pass
 
def Run_all_tests():
    my_setup()
    Use_a()
    Use_b()
 
if __name__ == '__main__':
 
#    t = timeit.timeit( 'Run_all_tests()', 'from __main__ import Run_all_tests', number=1 )
#    print dir( t )
#    print t
 
    t = timeit.Timer( 'Run_all_tests()', 'from __main__ import Run_all_tests' )
    print t.timeit()

When diving in, cProfile may come in handy.

import cProfile
 
def my_function():
    # Complicated stuff
    pass
 
if __name__ == '__main__':
    cProfile.run( "my_function()" )

Dynamically Calculating Column Size

Line up columns

data = '''\
234 127 34 23 45567
23 12 4 4 45
23456 2 1 444 567'''
 
# Split input data by row and then on spaces
rows = [ line.strip().split(' ') for line in data.split('\n') ]
 
# Reorganize data by columns
cols = zip(*rows)
 
# Compute column widths by taking maximum length of values per column
col_widths = [ max(len(value) for value in col) for col in cols ]
 
# Create a suitable format string
format = ' '.join(['%%%ds' % width for width in col_widths ])
 
# Print each row using the computed format
for row in rows:
  print format % tuple(row)

Which outputs:

  234 127 34  23 45567
   23  12  4   4    45
23456   2  1 444   567

Also, here's a nice summary of string formatting in Python.

Different Types of Objects

class A:
    """ Old, obsolete. """
    def __init__(self):
        self.__m_x = 0
 
    def getx(self):
        return self.__m_x
 
    def setx(self, x):
        if x < 0: x = 0
        self.__m_x = x
 
    x = property( getx, setx )
 
class B:
    """ Old; very small, was for for multitudes of objects. """
    __slots__ = [ "__m_x" ]
    def __init__(self):
        self.__m_x = 0
 
    def getx(self):
        return self.__m_x
 
    def setx(self, x):
        if x < 0: x = 0
        self.__m_x = x
 
    x = property( getx, setx )        
 
class C(object):
    """ New, reccommended. """
    def __init__(self):
        self.__x = 0
 
    def getx(self):
        return self.__x
 
    def setx(self, x):
        if x < 0: x = 0
        self.__x = x
 
    x = property( getx, setx )

filelock

Evan Fosmark has a filelock module. But here's a quick and dirty implementation of a lock that uses the current file:

import os
import fcntl
import inspect
# Maybe use os.path.abspath(__file__) ?
with open(os.path.abspath inspect.getfile(inspect.currentframe())), 'r') as f:
    try:
        fcntl.flock(f, fcntl.LOCK_EX)
        call_that_cannot_be_concurrent()
    finally:
        fcntl.flock(f, fcntl.LOCK_UN)

Fibonacci Generator with Itertools

import itertools
 
def fib(n):
    """Print a Fibonacci series up to n."""
    a, b = 0, 1
    while True:
        yield a
        b = a + b
        yield b
        a = a + b
 
if __name__ == '__main__': 
    for x in itertools.islice(fib(), 5):
        print x
 
#    for i in range( 5 ):
#        print i, fib( i )

TODO

TODO Link to my tips from LiveJournal and GMail, and why I chose which timing modules.

python/python.txt · Last modified: 2015/04/19 14:05 by dblume
 
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